Y24-115 - Add a reusable workflow to automate the generation of issue title numbers

[SHIV5T3R]

Closes sanger/General-Backlog-Items#406

Changes proposed in this pull request

• A new workflow that will run when an issue is opened which will increment an organisation variable that tracks the number of issues across the organisation.
• The incremented value will be used to prefix the issue title when an issue number, e.g: Y24-056, Y24-1042.

Instructions for Reviewers

[All PRs] - Confirm PR template filled
[Feature Branches] - Review code
[Production Merges to main]
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    - Check for debug code
    - Check version